[ty] suppress autocomplete suggestions during variable binding (#21549)

Statements such as `def foo(p<CURSOR>`,
`def foo[T<CURSOR>` and `for foo<CURSOR>`
should not generate any suggestions as these
cases are introducing new names.

If it's not possible to determine that suggestions should be omitted
using token matching in an easy way, we turn
to traversing the AST to determine the context.

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## Summary

Fixes https://github.com/astral-sh/ty/issues/1563

It keeps using the existing token matching pattern for the easy cases
(nothing typed and most recent token is a definition token) and
fallbacks to AST traveral for the slightly more difficult cases where
token matching becomes difficult and error prone.

<!-- What's the purpose of the change? What does it do, and why? -->

## Test Plan
New test cases and sanity-checking in the ty playground
<!-- How was it tested? -->

crates/ty_ide/src/completion.rs

@@ -8,7 +8,7 @@ use ruff_python_ast as ast;
 use ruff_python_ast::name::Name;
 use ruff_python_codegen::Stylist;
 use ruff_python_parser::{Token, TokenAt, TokenKind, Tokens};
-use ruff_text_size::{Ranged, TextRange, TextSize};
+use ruff_text_size::{Ranged, TextLen, TextRange, TextSize};
 use ty_python_semantic::{
     Completion as SemanticCompletion, ModuleName, NameKind, SemanticModel,
     types::{CycleDetector, Type},
@@ -329,7 +329,7 @@ pub fn completion<'db>(
     let tokens = tokens_start_before(parsed.tokens(), offset);
     let typed = find_typed_text(db, file, &parsed, offset);
 
-    if is_in_no_completions_place(db, file, tokens, typed.as_deref()) {
+    if is_in_no_completions_place(db, file, &parsed, offset, tokens, typed.as_deref()) {
         return vec![];
     }
 
@@ -1270,10 +1270,14 @@ fn find_typed_text(
 fn is_in_no_completions_place(
     db: &dyn Db,
     file: File,
+    parsed: &ParsedModuleRef,
+    offset: TextSize,
     tokens: &[Token],
     typed: Option<&str>,
 ) -> bool {
-    is_in_comment(tokens) || is_in_string(tokens) || is_in_definition_place(db, file, tokens, typed)
+    is_in_comment(tokens)
+        || is_in_string(tokens)
+        || is_in_definition_place(db, file, parsed, offset, tokens, typed)
 }
 
 /// Whether the last token is within a comment or not.
@@ -1296,11 +1300,18 @@ fn is_in_string(tokens: &[Token]) -> bool {
 
 /// Returns true when the tokens indicate that the definition of a new
 /// name is being introduced at the end.
-fn is_in_definition_place(db: &dyn Db, file: File, tokens: &[Token], typed: Option<&str>) -> bool {
+fn is_in_definition_place(
+    db: &dyn Db,
+    file: File,
+    parsed: &ParsedModuleRef,
+    offset: TextSize,
+    tokens: &[Token],
+    typed: Option<&str>,
+) -> bool {
     fn is_definition_token(token: &Token) -> bool {
         matches!(
             token.kind(),
-            TokenKind::Def | TokenKind::Class | TokenKind::Type | TokenKind::As
+            TokenKind::Def | TokenKind::Class | TokenKind::Type | TokenKind::As | TokenKind::For
         )
     }
 
@@ -1314,11 +1325,37 @@ fn is_in_definition_place(db: &dyn Db, file: File, tokens: &[Token], typed: Opti
             false
         }
     };
-    match tokens {
+    if match tokens {
         [.., penultimate, _] if typed.is_some() => is_definition_keyword(penultimate),
         [.., last] if typed.is_none() => is_definition_keyword(last),
         _ => false,
+    } {
+        return true;
     }
+    // Analyze the AST if token matching is insufficient
+    // to determine if we're inside a name definition.
+    is_in_variable_binding(parsed, offset, typed)
+}
+
+/// Returns true when the cursor sits on a binding statement.
+/// E.g. naming a parameter, type parameter, or `for` <name>).
+fn is_in_variable_binding(parsed: &ParsedModuleRef, offset: TextSize, typed: Option<&str>) -> bool {
+    let range = if let Some(typed) = typed {
+        let start = offset - typed.text_len();
+        TextRange::new(start, offset)
+    } else {
+        TextRange::empty(offset)
+    };
+
+    let covering = covering_node(parsed.syntax().into(), range);
+    covering.ancestors().any(|node| match node {
+        ast::AnyNodeRef::Parameter(param) => param.name.range.contains_range(range),
+        ast::AnyNodeRef::TypeParamTypeVar(type_param) => {
+            type_param.name.range.contains_range(range)
+        }
+        ast::AnyNodeRef::StmtFor(stmt_for) => stmt_for.target.range().contains_range(range),
+        _ => false,
+    })
 }
 
 /// Order completions according to the following rules:
@@ -5174,6 +5211,96 @@ match status:
         );
     }
 
+    #[test]
+    fn no_completions_in_empty_for_variable_binding() {
+        let builder = completion_test_builder(
+            "\
+for <CURSOR>
+",
+        );
+        assert_snapshot!(
+            builder.build().snapshot(),
+            @"<No completions found>",
+        );
+    }
+
+    #[test]
+    fn no_completions_in_for_variable_binding() {
+        let builder = completion_test_builder(
+            "\
+for foo<CURSOR>
+",
+        );
+        assert_snapshot!(
+            builder.build().snapshot(),
+            @"<No completions found>",
+        );
+    }
+
+    #[test]
+    fn no_completions_in_for_tuple_variable_binding() {
+        let builder = completion_test_builder(
+            "\
+for foo, bar<CURSOR>
+",
+        );
+        assert_snapshot!(
+            builder.build().snapshot(),
+            @"<No completions found>",
+        );
+    }
+
+    #[test]
+    fn no_completions_in_function_param() {
+        let builder = completion_test_builder(
+            "\
+def foo(p<CURSOR>
+",
+        );
+        assert_snapshot!(
+            builder.build().snapshot(),
+            @"<No completions found>",
+        );
+    }
+
+    #[test]
+    fn no_completions_in_function_type_param() {
+        let builder = completion_test_builder(
+            "\
+def foo[T<CURSOR>]
+",
+        );
+        assert_snapshot!(
+            builder.build().snapshot(),
+            @"<No completions found>",
+        );
+    }
+
+    #[test]
+    fn completions_in_function_type_param_bound() {
+        completion_test_builder(
+            "\
+def foo[T: s<CURSOR>]
+",
+        )
+        .build()
+        .contains("str");
+    }
+
+    #[test]
+    fn completions_in_function_param_type_annotation() {
+        // Ensure that completions are no longer
+        // suppressed when have left the name
+        // definition block.
+        completion_test_builder(
+            "\
+def foo(param: s<CURSOR>)
+",
+        )
+        .build()
+        .contains("str");
+    }
+
     #[test]
     fn favour_symbols_currently_imported() {
         let snapshot = CursorTest::builder()